1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as a copying machine or printer.
2. Related Background Art
Conventional image forming apparatuses include an electrophotographic copying machine, laser beam printer, and laser facsimile apparatus.
In the electrophotographic image forming apparatus, the electrification and exposure characteristics of the photosensitive drum vary depending on environmental conditions or changes over time. The image quality must be adjusted by properly adjusting the electrification potential (dark portion potential) by the electrification means and the exposure potential (light portion potential) by the exposure means in activation of the apparatus or before image formation.
Japanese Patent Application Laid-Open No. 50-81662 discloses a simple control method using linear approximation. In the first control executed in activation or prior to image formation after a long idle time, the electrification means is driven by two current values I1 and I2 to measure electrification potentials VD1 and VD2 of the photosensitive drum at the respective current values. A control coefficient (slope of a straight line I-V) is obtained from these values, and a current value I3 which provides a target potential is calculated based on the control coefficient. In the second control executed prior to image formation after a short idle time, a current value I4 which provides a target potential is easily calculated using the current value I3 and control coefficient obtained in advance.
This control method must perform two or more control operations in the first control even if the electrification and exposure characteristics of the photosensitive drum do not vary. The loss of time undesirably delays the first printing time.
The control method assumes that the current value and electrification potential are linear, and that the characteristics of the photosensitive drum vary linearly.
However, the electrification characteristic (exposure characteristic) does not always vary linearly depending on changes of the photosensitive drum over time or environmental changes. In this case, the validity of the default current values I1 and I2 is low, and an error becomes large between a potential calculated by linear approximation and an actual target potential. Especially in the second control using the previously calculated control coefficient, the reliability of the control coefficient is low. Accordingly, the number of control operations increases, and the convergence precision may decrease.
It is an object of the present invention to provide an image forming apparatus capable of decreasing the number of electrification potential control operations.
It is another object of the present invention to provide an image forming apparatus capable of obtaining a high-reliability control coefficient.
It is still another object of the present invention to provide an image forming apparatus comprising:
an image bearing member which bears an electrostatic image;
electrification means for electrifying the image bearing member to a predetermined potential;
exposure means for exposing the image bearing member electrified by the electrification means to an image;
potential detection means for detecting a potential of the image bearing member; and
determination means for performing calculation using a control coefficient for the potential detected by the potential detection means, and determining an electrification bias value,
wherein the determination means corrects the control coefficient to determine an electrification bias when the detected potential upon electrification at the electrification bias obtained by calculation falls outside a predetermined range.
The above object and other objects, features, and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments in conjunction with the accompanying drawings.